.Extend comes from the Latin tendere, to stretch; expand comes from Latin pandere, to spread.

In both physical and metaphorical use, extend tends to be used for enlargement along a single axis or in a single direction (my boundary extends from A to B), where expand tends to mean spreading out in many directions (gas expands to fill a vacuum). The latter is the sense meant in this question.._________________Native English teacher at Mister Micawber's